There has been a lot of discussion about what the relationship between TEI
XML and some forms of HTML might be.
Such discussion is great, because HTML is likely to remain the dominant
textual display form for years to come. We need to play nicely with HTML
(and ePub, which is essentially a carrier for HTML).
However, there are clear semantic differences between HTML and TEI.
All the semantics additions to HTML consist of tagging either the entire
document or some portion of the document text with additional semantics
over and above the baseline semantics of HTML. This additive model is
great, because it allows tools to understand HTML without understanding
all the semantic additions---tools which rely on just the semantics of HTML
can safely ignore all the rest.
Alas, this additive approach assumes only addition.
When you have tei:ab defined using phrases like "...analogous to, but
without the semantic baggage of, a paragraph", you have wording that is
categorically not representable in any of the HTML semantic systems
that I'm aware of.

The span element doesn't mean anything on its own, but
can be useful when used together with the global attributes, e.g.
class, lang, or dir. It represents its children.

If you add Microdata to a div, you would be able to represent tei:ab
without polluting it with some other HTML tag's explicitness (though
in this case, <div>, by being an anonymous block, would I
think already be equivalent).

(The difference between div and span, by the way, is that
<div> is rendered by default in a browser as a "block"
element, meaning that line breaks will be added on either side of
it, whereas a <span> is rendered without any formatting by
default, being inline (though these default behaviors can be
overridden completely by a CSS stylesheet where purely formatting
decisions are to be controlled--e.g., turning a div into an inline
element, or a span into a block element).)